There are lots of studies around it. I can dig out a few via google but you can probably find more by searching
https://www.wsj.com/articles/the-art-of-suggested-donation-amounts-1481511960
https://hbr.org/2016/12/when-and-how-fundraisers-should-suggest-how-much-to-donate
https://www.wsj.com/articles/the-art-of-suggested-donation-amounts-1481511960
https://hbr.org/2016/12/when-and-how-fundraisers-should-suggest-how-much-to-donate
As a result, the effects of defaults on total funds raised are different when targeting loyal donors. Donors who strongly identify with the cause or organization and have a history of giving may have already decided whether to participate, but they may be unsure about how much to give. For these loyal donors, a high default amount can result in more funds raised.